Molecular gas in AzTEC/C159: a star-forming disk galaxy 1.3 Gyr after the Big Bang

Abstract

We studied the molecular gas properties of AzTEC/C159, a star-forming disk galaxy at z = 4.567, in order to better constrain the nature of the high-redshift end of the sub-mm selected galaxy (SMG) population. We secured 12 CO molecular line detections for the J=2→1 and J=5→4 transitions using the Karl G. Jansky Very Large Array (VLA) and the NOrthern Extended Millimeter Array (NOEMA) interferometer. The broad (FWHM∼ 750 km s −1 ) and tentative double-peaked profiles of both 12 CO lines are consistent with an extended molecular gas reservoir, which is distributed in a rotating disk as previously revealed from [CII] 158 µm line observations. Based on the 12 CO(2→1) emission line we derived L CO = (3.4 ± 0.6) × 10 10 K km s −1 pc 2 , that yields a molecular gas mass of M H 2 (α CO /4.3) = (1.5 ± 0.3) × 10 11 M and unveils a gas-rich system with µ gas (α CO /4.3) ≡ M H 2 /M = 3.3 ± 0.7. The extreme star formation efficiency (SFE) of AzTEC/C159, parametrized by the ratio L IR /L CO = (216 ± 80) L (K km s −1 pc 2 ) −1 , is comparable to merger-driven starbursts such as local ultra-luminous infrared galaxies (ULIRGs) and SMGs. Likewise, the 12 CO(5→4)/CO(2→1) line brightness temperature ratio of r 52 = 0.55 ± 0.15 is consistent with high excitation conditions, similar to that observed in SMGs. Based on mass budget considerations we constrained the value for the L CO -H 2 mass conversion factor in AzTEC/C159, i.e. α CO = 3.9 +2.7 −1.3 M K −1 km −1 s pc −2 , that is consistent with a self-gravitating molecular gas distribution as observed in local star-forming disk galaxies. Cold gas streams from cosmological filaments might be fueling a gravitationally unstable gas-rich disk in AzTEC/C159, which breaks into giant clumps forming stars as efficiently as in merger-driven systems and generate high gas excitation. These results support the evolutionary connection between AzTEC/C159-like systems and massive quiescent disk galaxies at z ∼ 2.

The Redshift and Star Formation Mode of AzTEC2: A Pair of Massive Galaxies at z = 4.63

“…The lensing-corrected SFR of AzTEC2-B (710 ± 35 M yr −1 ) is consistent with the average for SMGs at similar redshifts (∼ 800 M yr −1 ; e.g., Smolčić et al 2015;Gómez-Guijarro et al 2018;Jiménez-Andrade et al 2018;Magnelli et al 2019). The extreme IR-based SFR of AzTEC2-A (SFR=1920 ± 100 M yr −1 ) is comparable to those of the massive, star-forming disks GN20 and AzTEC1 at z ∼ 4.5 Magnelli et al 2012;Tadaki et al 2018).…”

“…6 the Σ gas range given by α CO varying from 0.8 to 4.3 M K −1 km −1 s pc −2 . We also recall that in deriving L CO(1→0) , and hence M gas , we have assumed typical SMG-like gas excitation conditions, as previously ob-served in other massive, highly star-forming rotating disks like GN20 and AzTEC/C159 Hodge et al 2012;Jiménez-Andrade et al 2018). By combining our Σ gas and Σ SFR estimates we find that AzTEC2-A lies at the upper end of the Kennicutt-Schmidt relation for typical SFGs (Fig.…”

“…Then, we derive v rot /σ 5, indicating that AzTEC2-A is rather an unperturbed, rotation-dominated disk that resembles the v rot /σ ratio of more evolved disk galaxies at z ∼ 1 (e.g., Di Teodoro et al 2016, and references therein). This finding, therefore, provides more evidence of kinematically mature disks that can be found at even z ∼ 4.5, such as GN20 (Hodge et al 2012), ALESS 73.1 (De Breuck et al 2014), AzTEC/C159, J1000+0234, Jiménez-Andrade et al 2018) and AzTEC1 (Tadaki et al 2018).…”

“…In order to approximate the galaxy-averaged molecular gas surface density (Σ gas ≡ M gas /(2πR 2 eff )), we use our M gas estimate based on L CO(1→0) ( §4.3) and α CO = 2.5 M K −1 km −1 s pc −2 . Although the rotation-dominated gas disk of AzTEC2-A favors a higher α CO value (e.g., Papadopoulos et al 2012a,b;Jiménez-Andrade et al 2018), we re-…”

“…The gas depletion time-scale (τ gas ) of AzTEC2-A, in particular, exhibit a mild excess with respect to the average for SMGs at similar redshifts (τ gas ∼ 45 Myr; Aravena et al 2016), but resembles the one of the massive, star-forming disk galaxies GN20, AzTEC/C159 and AzTEC1 at z The diamonds represent the parameter space covered by the SMGs reported in . Large symbols correspond to the SMGs studied here and the massive, rotating disk galaxies at z ∼ 4.5: GN20, AzTEC1, and AzTEC/C159 Hodge et al 2012;Jones et al 2017;Jiménez-Andrade et al 2018;Tadaki et al 2018). In the case of AzTEC2-A and AzTEC2-B, we use lensing-corrected luminosities.…”